Speaker
Description
Atomic hydrogen has an ideal level scheme for an optical clock. Since there is no 1P state, the 2S state can neither decay nor be excited with a single photon dipole transition, at least not in a field-free environment. This has three important advantages: The lifetime of the 2S is very long leading to natural line width of 1.3Hz. This is a good value for an optical clock. Because the 1S-2S clock transition at 2466THz is excited with two photons, the required laser operates at 243nm rather than at 121.5nm (Lyman-alpha). Moreover, the two-photon excitation can be arranged such that it is free of the Doppler effect in first order. This also implies that only moderately low temperatures and no strong confinement (Lamb-Dicke regime) of the trapped atoms is required. The magic wavelength [1] for the 1S-2S transition is at a convenient value of 515nm [2]. High power narrow band lasers are readily availa-ble by frequency doubled Yb-based lasers.
Obviously, the main showstopper is the required Lyman-alpha laser for cooling. Cooling atomic hydrogen that is already trapped has been achieved with pulsed Lyman-alpha lasers. These are not too difficult to realize with a low repetition rate and hence a large pulse energy to enhance the required non-linear frequency conversion. For loading the trap, a continuous wave or a high repetition rate laser with sufficient power would be required. This has not been possible so far. While magnetic trapping of hydrogen and anti-hydrogen have been demonstrated, we would like to avoid strong magnetic fields in precision experiments because of the large Zeeman shifts (Bohr’s magneton is 14GHz/Tesla). Moreover, due to the low atomic mass and the large photon momentum, cooling on the 1S-2P transition would be rather inefficient with the Doppler and recoil limit as high as 2.39mK and 1.29mK respectively.
A number of proposals have been published to circumvent the Lyman-alpha laser [3,4,5,6,7]. You should not miss this presentation if you want to find out about our approach. It uses the selection of the slow tail of velocities from a thermal beam and the photon recoil by an induced decay of the meta-stable 2S state inside an optical dipole trap. In contrast to laser cooling, this method works better the lower the atomic mass and the larger the photon recoil. Besides of improving the measured transition frequencies, trapped atomic hydrogen could eventually be the motivation to redefine the SI second in terms of the Rydberg constant. This would remove the last remaining object in the definitions of the SI which is otherwise based defined values of physical constants (c, h and e).
[1] H.Katori, Proceedings of the 6th Symposium on Frequency Standards and Metrology, University of St Andrews, Fife, Scotland 9–14 September 2001.
[2] C.M.Adhikari, A.Kawasaki, and U.D.Jentschura, Phys. Rev. A 94, 032510 (2016).
[3] R.deCarvalho, N.Brahms, B.Newman, J.M.Doyle, D.Kleppner, and T.Greytak, Can. J. Phys. 83, 293 (2005).
[4] I.C.Lane, Phys. Rev. A 92, 022511 (2015).
[5] R.Côté, M.J.Jamieson, Z-C.Yan, N.Geum, G.H.Jeung, and A.Dalgarno, Phys. Rev. Lett. 84, 2806 (2000).
[6] S.F.Vázquez-Carson, Q.Sun, J.Dai, D.Mitra, and T.Zelevinsky, New J. Phys. 24 083006 (2022).
[7] S.A.Jones, New J. Phys. 24 023016 (2022).
